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© The Author(s), under exclusive license to Springer Nature Switzerland AG 2021
J. P. Zehr, D. G. Capone, Marine Nitrogen Fixation,
https://doi.org/10.1007/978-3-030-67746-6_4
Chapter 4
Microorganisms and Habitats
4.1 The Surface Ocean (Pelagic)
Since the oceans cover almost ¾ of the Earth’s surface, N 2 fixation in the water
column, particularly in the well-lit surface ocean supported by photosynthesis, can
be a major N input for the oceans (Gruber 2008) and even the globe (Galloway
2003). The pelagic zones of the oceans range from shallow coastal seas, to the welllit euphotic zone and the deep, dark cold waters of the mesopelagic and subeuphotic
zone. These habitats also range in nutrient availability (nutrient poor central gyres
versus nutrient rich upwelling and coastal zones) and O 2 concentration (e.g. Oxygen
Deficient Zones or ODZs), and thus provide diverse habitats with a wide range of
N 2 fixation rates and diazotroph diversity. Discrete ecological provinces of the sea
with respect to physical and chemical (i.e. nutrients) characteristics have been
established by Longhurst (2007). In the marine environment, in contrast to the terrestrial environment, unicellular algae dominate photosynthesis, and multicellular
plants (seaweeds, sea grasses and marshland grasses) are largely relegated to ocean
margins (Fig. 4.1). Thus, cyanobacteria (and possibly heterotrophic bacteria), freeliving or symbiotic with unicellular microorganisms, are the major diazotrophs in
the water column of the open ocean. Much is known about the diverse microorganisms in the surface ocean.
Cyanobacteria
Surface waters range from cold polar seas to warm tropical and subtropical waters,
from saline to low salinity estuaries, and from coastal to open ocean. Very different
microbial communities inhabit these divergent ecosystems (see Chap. 7).
Filamentous, heterocyst-forming cyanobacteria such as Nodularia and
Aphanizomenon (Fig. 4.2f, g) are often found and sometimes form thick blooms in
© The Author(s), under exclusive license to Springer Nature Switzerland AG 2021
J. P. Zehr, D. G. Capone, Marine Nitrogen Fixation,
https://doi.org/10.1007/978-3-030-67746-6_4
Chapter 4
Microorganisms and Habitats
4.1 The Surface Ocean (Pelagic)
Since the oceans cover almost ¾ of the Earth’s surface, N 2 fixation in the water
column, particularly in the well-lit surface ocean supported by photosynthesis, can
be a major N input for the oceans (Gruber 2008) and even the globe (Galloway
2003). The pelagic zones of the oceans range from shallow coastal seas, to the welllit euphotic zone and the deep, dark cold waters of the mesopelagic and subeuphotic
zone. These habitats also range in nutrient availability (nutrient poor central gyres
versus nutrient rich upwelling and coastal zones) and O 2 concentration (e.g. Oxygen
Deficient Zones or ODZs), and thus provide diverse habitats with a wide range of
N 2 fixation rates and diazotroph diversity. Discrete ecological provinces of the sea
with respect to physical and chemical (i.e. nutrients) characteristics have been
established by Longhurst (2007). In the marine environment, in contrast to the terrestrial environment, unicellular algae dominate photosynthesis, and multicellular
plants (seaweeds, sea grasses and marshland grasses) are largely relegated to ocean
margins (Fig. 4.1). Thus, cyanobacteria (and possibly heterotrophic bacteria), freeliving or symbiotic with unicellular microorganisms, are the major diazotrophs in
the water column of the open ocean. Much is known about the diverse microorganisms in the surface ocean.
Cyanobacteria
Surface waters range from cold polar seas to warm tropical and subtropical waters,
from saline to low salinity estuaries, and from coastal to open ocean. Very different
microbial communities inhabit these divergent ecosystems (see Chap. 7).
Filamentous, heterocyst-forming cyanobacteria such as Nodularia and
Aphanizomenon (Fig. 4.2f, g) are often found and sometimes form thick blooms in